This applicant previously invented a constant flow valve serving as an automatic regulating valve intended to supplement a constant flow rate control which provided functions of controlling a constant flow rate and optionally setting a controlled flow rate, and further, acted as a stop valve which opened and closed, and applied for a Utility Model Right for the valve. This application was published as Japanese Unexamined Utility Model Publication No. 62-96173, under the title of "Constant Flow Valve".
The "Constant Flow Valve" disclosed in the above Japanese Unexamined Utility Model Publication No. 62-96173 comprises a stop-valve-type body having an opening tapered from a downstream side toward an upstream side thereof. The opening and a valve plug fitted to a lower end of a piston valve form an orifice, and a pressure differential created between a point in front of and a point behind the orifice is introduced through a pressure introducing port provided inside the piston valve, and through a pressure introducing port provided in a spring seat, into primary and secondary pressure chambers defined by a piston in a cylinder, thereby amplifying the pressure difference and operating the piston valve in response to the pressure differential to thus maintain a constant flow rate. The cylinder is movable back and forth along an axis of the opening so that an initial operation position of the valve plug with respect to the opening is changeable to enable an optional setting of the controlled flow rate. A lower end face of the cylinder has a sealing portion, which is pressed against a valve seat to completely stop a fluid flow. In this way, unlike conventional valves, this constant flow valve simultaneously has three functions of "controlling a constant flow rate", "optionally setting a controlled flow rate", and "serving as a stop valve". Inside the cylinder, where a fluid usually stagnates, the piston valve can freely slide to cause the fluid to leak from the primary pressure chamber to the secondary pressure chamber, and then to an outlet passage, to thereby prevent an occurrence of "dead water". As a result, this valve has a vital utility in the chemical industry and semiconductor manufacturing field.
Nevertheless, because this type of valve leaks a fluid from the primary pressure chamber to the secondary pressure chamber, and then to the outlet passage, it has the following two drawbacks, (1) part of the controlled flow leaks to the outlet passage, and thus there is little control of a very small flow rate and there is a limit on the range of the controlled flow rates, the range of this and other known valves is generally 4:1 to 8:1 at present, and (2) if a line involves a lot of dust, sliding parts of the piston are abraded by the dust, due to the leak of a fluid from a first pressure chamber to a second chamber, and thus malfunctions often occur.